Sedimentary Record of Impact Events in Spain

Geological Society of America Special Paper 356 2002

Sedimentary record of impact events in Spain

Enrique Dıáz-Martıńez* Enrique Sanz-Rubio Jesu´s Martıńez-Frıás Centro de Astrobiologıá Consejo Superior de Investigaciones Cientı´ficas—Instituto Nacional de Tećnica Aeroespacial, Carretera, Torrejoń-Ajalvir kilo´metro 4, 28850 Torrejoń de Ardoz, Madrid, Spain

ABSTRACT

A review of the evidence of meteorite-impact events in the sedimentary record of Spain reveals that the only proven impact-related bed is the clay layer at the Cretaceous-Tertiary boundary (at Zumaya and Sopelana in the Bay of Biscay region, and at Caravaca, Agost, and Alamedilla in the Betic Cordilleras). Other deposits previously proposed as impact related can now be rejected, or are dubious and still debated. These include the Pelarda Formation, alleged to represent proximal ejecta from the Azuara structure; the Paleocene-Eocene boundary near Zumaya (western Pyrenees) and Alamedilla (Betic Cordillera); and the Arroyofrı´o Oolite Bed, which has been alleged as distal ejecta of an unknown Callovian-Oxfordian impact event. The scarcity of evidence for meteorite-impact events in the sedimentary record is possibly due to a lack of detailed studies. We propose several sedimentary units that could potentially be related to impact events, and where future research should focus.

INTRODUCTION DISTAL RECORD OF IMPACT EVENTS

The sedimentary record of Spain presents evidence for at The evidence from sedimentary units to be considered as least one impact event, as well as a number of units of potential distal impact ejecta may consist of geochemical anomalies of impact clastic origin (some of which are currently under inves- elements and isotopes (e.g., Ir, 187Os/188Os), the presence of tigation). In this chapter we summarize and review the infor- impact ejecta in the sediments (e.g., shocked minerals, micro- mation available about the sedimentary record of meteorite im- tektites, or spherules), or tsunami deposits (Montanari and Koe- pacts in Spain (Fig. 1), most of it published in Spanish journals. berl, 2000). Evidence for distal impact ejecta in the sedimentary In addition, we propose several stratigraphic units with poten- record of Spain has been proposed in relation with the Dogger- tial for future research. In this contribution we attempt (1) to Malm, Cretaceous-Tertiary (K-T), and Paleocene-Eocene bring to the attention of the international community recent and boundaries, as discussed in the following. In brief, the only ongoing research relating to the sedimentary record of impact proven distal record of an impact event in Spain is found at the events in Spain, (2) to review the current knowledge and inter- K-T boundary. Studies of the Paleocene-Eocene extinction pretation of units previously proposed as related to impacts, and event in Spanish sections have shown major changes in pale- (3) to promote new research within selected units to evaluate oceanographic conditions, the causes of which are still debated, the possibility of an impact origin. but an impact origin remains only probable. Future work should

*E-mail: [email protected]

Dıáz-Martıńez, E., Sanz-Rubio, E., and Martıńez-Frıás, J., 2002, Sedimentary record of impact events in Spain, in Koeberl, C., and MacLeod, K.G., eds., Catastrophic Events and Mass Extinctions: Impacts and Beyond: Boulder, Colorado, Geological Society of America Special Paper 356, p. 551–562. 551 552 E. Dıáz-Martıńez, E. Sanz-Rubio, and J. Martıńez-Frıás

spherules, and Fe-Mn Bacterial-fungal stromatolites. According to Sepkoski (1996), the Callovian-Oxfordian interstage boundary coincides with a Ͼ20% extinction of marine fossil genera. Relatively high levels of extinction percentages are reported throughout the Middle and Upper Jurassic, although no clearly defined peak can be identified (Sepkoski, 1996). In any case, these values are higher than the percentage of extinction coinciding with other known large impact events, such as the late Eocene Chesapeake and Popigai events. No proven impact structure or impact signatures have been found at or near the Middle-Upper Jurassic boundary anywhere in the world that could be related to a large impact event (Montanari and Koe- berl, 2000). Therefore, any evidence in the sedimentary record that is not unequivocal should be carefully considered before a cosmic origin is inferred. The unit studied by Meleńdez et al. (1987) is known as the Arroyofrıó Oolite Bed, a thin discontinuous bed at the top of the Chelva Formation and directly below the Ya´tova Forma- tion; both of these formations are shallow-marine carbonate units found at many sections throughout the eastern branch of the Iberian Range (Go´mez, 1979; Aurell and Meleńdez, 1990). The Arroyofrıó Oolite Bed is a condensed unit, Ͻ1 m thick, Figure 1. Localities mentioned in text: 1, Alamedilla; 2, Agost; 3, consisting of wackestone and packstone with iron oolites and Azuara and Lećera; 4, Caravaca; 5, Nazare´; 6, Osinaga and Musquiz; bioclasts. Bioclasts include ammonites, planktonic foraminif- 7, Pozuel del Campo; 8, Ricla; 9, Sopelana; 10, Valdelacasa and Na- era, brachiopods, and belemnites, which were dated as mid- valpino; 11, Valverde del Camino; 12, Zumaya. Callovian to early Oxfordian by Ramajo et al. (2000). Workers in Spanish basins usually interpret the Arroyofrıó Oolite Bed focus on high-resolution studies on marine stratigraphic sec- as a result of a series of punctuated subaerial exposure and tions comprising critical boundaries related to major biotic and/ transgressive events resulting in condensed carbonate sedimen- or climatic events (e.g., Triassic-Jurassic, Paleocene-Eocene, tation in a shallow-marine setting near local paleogeographic late Eocene). highs, under the influence of local currents and regional tectonic or tectono-eustatic controls (Aurell et al., 1990, 1994; Aurell, Middle-Upper Jurassic boundary 1991; Ramajo and Aurell, 1997; Ramajo et al., 2000). Meleńdez et al. (1987) mentioned sedimentological and The Middle-Upper Jurassic boundary throughout many biostratigraphic evidence for hardground and hiatus develop- peri-Atlantic basins is associated with a stratigraphic gap span- ment, together with local (parautochthonous) resedimentation. ning at least the upper Callovian–lower Oxfordian interval Their studies revealed geochemical anomalies of certain sid- (three ammonite biozones), although, in places, the missing rec- erophile elements (Fe, Mn, Ni, Co). In some cases, Pt and Ir ord is much longer. On a regional scale, it is normally accepted were found in relatively high proportions. In our opinion, the Mn/Al (indicative of hydrothermal ם that a major tectono-eustatic event controlled this widespread high proportions of Fe stratigraphic boundary, usually including emersion and/or con- processes), in conjunction with the evidence for submarine cor- densed levels (Aurell, 1991; Aurell et al., 1994). These features rosion by acid waters, and the occasional presence of bacterial are recorded, among others, in the Lusitanian basin (west- stromatolites, point the geochemical anomalies being related to central Portugal), the Iberian, Catalonian, and Cantabrian basins shallow submarine hydrothermal vents and volcanic activity. (northeastern and northwestern Spain), Bourgogne and the Paris Based on the high concentration of Ni-Fe-rich spherules found basin (France), the Jura basin (Switzerland), and the Neuqueń at one locality (Ricla), Meleńdez et al. (1987) interpreted the basin (Argentina). volcanic and hydrothermal activity as triggered by the impact The Dogger-Malm boundary at Ricla and Pozuel del of a cosmic body. In their interpretation, the other phenomena Campo (Iberian Range; Fig. 1) presents several features that recorded at the boundary represent the effects of such an im- were interpreted by Meleńdez et al. (1987) as related to an pact. However, the evidence presented in favor of a cosmic impact event. This hypothesis was based on the presence of origin for the disconformity at the Middle-Upper Jurassic conspicuous geochemical anomalies (e.g., heavy metals and boundary in the Iberian Range is not unequivocal. Discussing platinum group elements [PGE]), volcanic and hydrothermal a recently discovered modern analogue for iron ooids and pi- activity, submarine corrosion, high concentration of iron-rich soids in a shallow-marine volcanic setting in Indonesia, Stures- Sedimentary record of impact events in Spain 553 son et al. (2000) demonstrated that iron ooids form by chemical marls and scarce turbiditic calcarenite beds rich in macrofora- precipitation of cryptocrystalline iron oxyhydroxides on avail- minifera (Usera et al., 2000). The K-T boundary is represented able grains on the seafloor, from seawater enriched with Fe, Al by a dark gray, 12-cm-thick clay layer that has a red-yellowish and Si. The enrichment can be the result of hydrothermal fluids, lamina, enriched in goethite and hematite, at the base (Usera et volcanic ash falling into shallow basins, or rapid weathering of al., 2000). This lamina contains impact evidence, such as spher- fresh volcanic rocks. More detailed research should be carried ules, isotopic changes, and anomalies of Ir, Co, Ni, Cr, and other out on the geochemistry of the spherules and PGE anomalies elements (Martıńez-Ruı´z et al., 1992a, 1997). Fe-oxide spher- found within the Arroyofrıó Oolite Bed before a possible cos- ules at Agost are more abundant than K-feldspar spherules, mic origin should be considered. some of the Fe-oxide spherules showing fibroradial and den- dritic textures (Martıńez-Ruı´z et al., 1997). The Danian record K-T boundary comprises mainly gray marl with some interbeds of marly limestones, but toward the top of the section, reddish colors are The Spanish sedimentary record presents good examples dominant. The sedimentary continuity of the section has been of continuous upper Maastrichtian sedimentary sequences, such demonstrated by biostratigraphic studies (Molina et al., 1996). as the sections at Agost in Alicante (Groot et al., 1989), Cara- Several models for the extinction of planktonic foraminifera vaca in Murcia, and Zumaya in the Bay of Biscay region (Smit have been proposed for the K-T transition at Agost, such as an and Romein, 1985) (Fig. 1). The Agost and Caravaca sections almost total catastrophic mass extinction (Smit, 1990), a grad- are in the Betic Cordillera (southeastern Spain), whereas the ual mass extinction (Canudo et al., 1991; Pardo et al., 1996), Bay of Biscay region includes Zumaya and other remarkable and a catastrophic mass extinction superposed onto a less- sections in Spain (Sopelana, Osinaga, Musquiz) and France (Bi- evident gradual trend (Molina et al., 1996). dart, Hendaye) (Fig. 1). Most of these sections are in pelagic The Caravaca section is located 4 km southwest of Cara- to hemipelagic facies and contain rich foraminiferal and nan- vaca (Murcia Province, southeastern Spain; Fig. 1) and consti- nofossil faunas and floras, insignificant amounts of macrofos- tutes one of the most complete and least disturbed K-T sections sils, and little or no evidence for hardgrounds or omission sur- in the world (Canudo et al., 1991; MacLeod and Keller, 1991). faces (Smit, 1999). High-resolution studies resulted in a Terminal Maastrichtian–basal Paleocene sediments at Caravaca magnetostratigraphic record for the Caravaca and Agost sec- were deposited in a middle bathyal environment (200–1000 m tions, but a reliable magnetostratigraphy for the Upper Creta- depth), as indicated by benthic foraminiferal assemblages (Coc- ceous of the Bay of Biscay region has not been established cioni and Galeotti, 1994). Cretaceous and Tertiary lithologies (Kate and Sprenger, 1993; Moreau et al., 1994). of the transition are dominantly marly. The K-T boundary clay Sections of the Betic Cordilleras. The Agost and Caravaca layer is a 7–10-cm-thick dark clay-marl bed. The upper part of sections occur in the peri-Mediterranean Alpine orogenic belt. the boundary clay layer is disturbed, and burrows several cen- They are among the most complete marine sections for the timeters in length have been described (Arinobu et al., 1999). K-T transition, in which the K-T boundary layer provides an A 1–2-mm-thick orange basal layer rich in goethite also has excellent record of the distal ejecta facies related to the Chi- high Ir and Os concentrations, in conjunction with V, Cr, Fe, cxulub impact (Groot et al., 1989; Martıńez-Ruı´z, 1994). Marl Ni, Zn, and As anomalies, and a high content of small spherules is their main lithology of both sections, which are composed (Smit and Hertogen, 1980; Smit and Klaver, 1981; Smit, 1982; calcite, quartz, and clay minerals. A clayey 2–3-mm-thick layer Smit and ten Kate, 1982; Smit and Romein, 1985; Schmitz, appears on top of the Maastrichtian, marking the K-T boundary. 1988; Martıńez-Ruı´z et al., 1992b). The spherules are mainly It is characterized by an abrupt decrease in carbonate and by made of K-feldspar, 0.1–0.8 mm in diameter, and were first an increase in clay mineral content (Ortega-Huertas et al., 1995; discovered at the K-T boundary clay of the Caravaca section Martıńez-Ruı´z et al., 1997). PGE anomalies and spherules are by Smit and Klaver (1981). Fe-oxide spherules are rare at Car- confined to the boundary layer (200–400 spherules/cm3), where avaca (Martıńez-Ruı´z et al., 1997). spherules and smectites are the main components, and there are Kaiho and Lamolda (1999) concluded, for the Caravaca minor amounts of illite and kaolinite (Smit, 1990; Ortega- section, that most planktonic foraminifera did not survive and Huertas et al., 1995; Martıńez-Ruı´z et al., 1997). The compo- abruptly became extinct at the K-T boundary, on the basis of sition of most of the spherules is K-feldspar and Fe-oxides, stable isotope and foraminiferal abundance determinations. In probably as a result of diagenetic alteration and replacement of addition, Arinobu et al. (1999) carried out a study of carbon precursor clinopyroxene, as interpreted from relict crystalline isotope stratigraphy and detected a spike of the pyrosynthetic textures (Martıńez-Ruı´z et al., 1997; Smit, 1999). polycyclic aromatic hydrocarbons (PAHs) at the Caravaca K-T The Agost section is located 1.5 km north of Agost (Ali- boundary. Arinobu et al. proposed that the combustion of ter- cante Province, southeastern Spain), and covers the Late Cre- restrial organic matter in massive global fires was the most taceous through middle Eocene record. The K-T transition is probable mechanism for the origin of these PAHs. represented by open sea deposits. The Maastrichtian record The Alamedilla section (Granada; Fig. 1) has been de- consists of light gray pelagic marls, and interbeds of calcareous scribed as the closest site to the Chicxulub crater (ϳ7000 km 554 E. Dıáz-Martıńez, E. Sanz-Rubio, and J. Martıńez-Frıás away) with an undisturbed ejecta layer (Smit, 1999). Droplets crokrystites altered to As-rich pyrite have been described from recently found at Alamedilla were interpreted as altered tektites, the Zumaya section (Smit, 1982; Smit and Romein, 1985; indicating that tektites end microkrystites may occur together Schmitz et al., 1997) and from the Sopelana section (Rocchia in the same ejecta layer (Smit, 1999). et al., 1990). Above the calcite vein, 7–8 cm of dark gray shales Sections of the Bay of Biscay region. Stratigraphic sec- occur, and 25 cm of gray marls forming the so-called boundary tions around the Bay of Biscay region are valuable for testing marls (Lamolda et al., 1988). hypotheses of K-T transition extinctions (Smit et al., 1987). The reasons for this are: (1) these sections are considered by micro- Paleocene-Eocene boundary paleontologists to be relatively complete (Smit et al., 1987), (2) they exhibit high sedimentation rates, resulting in increased The largest extinction event having affected the deep-sea resolution of the stratigraphy, (3) they were deposited in a pe- benthic foraminiferal fauna during the past 90 m.y. occurred in lagic, but nonturbiditic, environment, and (4) well-exposed out- the latest Paleocene (Schmitz et al., 1997). The Zumaya section crops along coastlines facilitate access for measuring sections (Bay of Biscay region) also contains one of the most expanded and collecting samples. All the sections contain a conformable and biostratigraphically complete Paleocene-Eocene transi- sequence of Upper Cretaceous and lower Tertiary marine strata tions, deposited in a middle or lower bathyal environment (Pu- that were deposited in the Basque-Cantabrian basin (Lamolda jalte et al., 1993). At Zumaya, the benthic extinction event et al., 1981). This basin is part of the continental margin of closely coincides with deposition of a clay interval that indi- northern Spain, and is mostly filled with Mesozoic rocks. The cates strong CaCO3 undersaturation (Canudo et al., 1995). Ap- most striking geological feature of the region is the great thick- proximately 15 m below the benthic extinction event, the sec- ness of its Mesozoic-Tertiary sequence, which exceeds 15 km tion is dominated by gray marls that underlie a 4 m thick, (Garcıá-Monde´jar et al., 1985). This basin was one of several mainly reddish-brown clay interval. The benthic extinction forming along the boundary of the European-Iberian plates dur- event occurs at the base of the clay interval. A transition from ing the Late Cretaceous (Ward, 1988). Although deposition of marls to limestone occurs above the clay interval. turbiditic sediments dominated from the Campanian to the early High-resolution d18O and d13C, calcareous nannofossil, Maastrichtian, the reduction of siliciclastic material influx and and planktic and benthic foraminifera studies showed that, be- basin-wide shallowing and regression during the late Maas- low the marl-clay transition, there is a 40–50-cm-thick interval trichtian resulted in limestone-marl rhythmites (Lamolda et al., that contains a detailed record of a gradual succession of faunal 1981). Immediately following the K-T boundary, there was an and geochemical events culminating in the benthic extinctions even more dramatic reduction in siliciclastic influx into the ba- (Schmitz et al., 1997). There is a significant Ir anomaly (133 sin, resulting in the deposition of pink coccolith limestones dur- ppt over a background of 38 ppt) in a 1-cm-thick, gray marl ing the Danian (Ward, 1988). The most representative and most layer ϳ40 cm below the base of the clay interval. Above the Ir studied sections for the K-T boundary are Zumaya (Gipuzkoa anomaly, a negative gradual excursion of d13C is developed in Province) and Sopelana (Bizkaia Province) (Fig. 1). In addition, a 40-cm-thick, glauconitic, greenish-brown marl bed. The re- the nearby sections of Bidart and Hendaye (south of France) lation of these anomalies to an impact event and its role re- are also well-known for the K-T transition in the same region. garding mass extinctions related to a Paleocene-Eocene (P-E) The Maastrichtian-Paleocene of the Zumaya section is the event are debatable. Schmitz et al. (1997) indicated that, if the most thoroughly studied of the Bay of Biscay sections. It is the Ir anomaly can be related to an impact event, it may not have thickest, best exposed, and least faulted section (Ward, 1988). been of any consequence for ongoing paleoceanographic A continuous section from the lower Campanian to the Eocene changes and later mass extinctions. is exposed along the coastal cliff west of Zumaya. The advan- Major biotic and geochemical changes have also been tages of the Zumaya section are (Lamolda et al., 1988): (1) shown in the P-E at Alamedilla (Granada, Spain), where the sedimentary continuity across the K-T boundary, (2) relative transition is marked by major faunal turnover in planktic fora- abundance of fossil remains through the Maastrichtian, (3) al- minifera, mass extinction in benthic foraminifera, negative d18O most complete absence of turbidites in the purple marls and excursion in benthic foraminifera, negative d13C excursion in limestones of late Maastrichtian and early Paleocene ages, (4) both planktic and benthic foraminifera, decrease in calcite pres- thickness of and high sedimentation rate for the transitional ervation, increase in detrital flux, and changes in clay mineral beds, and (5) absence of tectonism affecting the transitional composition (Lu et al., 1995). According to Montanari and beds. The section has no boundary clay, but the boundary layer Koeberl (2000), there is no relationship between the four or five at Zumaya is pyritic, and therefore easy to recognize (Wied- impact craters known that are roughly P-E age, and the P-E mann, 1988). The uppermost part of the Maastrichtian is com- benthic extinction and associated d13C shift. posed of several thin beds of green marls (1–5 cm thick), a sandy gray-brown bed, and then purple marls (Lamolda et al., PROXIMAL RECORD OF IMPACT EVENTS 1988). The K-T boundary is marked by a single or multiple calcite vein (of supergenic nature) 2–3 cm thick, with gray dark The Azuara structure (41Њ01ЈN, 00Њ55ЈW, Zaragoza prov- shale interbeds. Ir anomalies and spherules interpreted as mi- ince, northeastern Spain), ϳ30 km in diameter (Fig. 2), is the Sedimentary record of impact events in Spain 555

Figure 2. Geological sketch of Azuara structure and adjacent Calatayud-Mon- talbań and Ebro basins (modified after Corte´s and Casas-Saínz, 1996). Pelarda Formation is located to south of Azuara structure (circled in map; R). A distance of one radius from outlined structure has also been marked (2R). Fm.—Forma- tion.

only structure on the Iberian Peninsula for which an impact son et al., 1985, 1999; Ernstson and Claudıń, 1990; Ernstson origin has been formally proposed. Following its identification and Fiebag, 1992). Several lines of evidence based on the sed- in the 1980s, a strong debate arose about either a tectonic or an imentary and structural evolution of the Azuara area, as well as impact-induced origin for this structure (e.g., Ernstson and Fie- that of the Iberian Range and the Ebro basin, were presented bag, 1992; Aurell et al., 1993). The controversy remains, al- against the hypothesis of a meteorite impact (Aurell et al., though the arguments in favor of the impact hypothesis are 1993), alternative interpretations being proposed for the criteria gradually being rejected, because most of the evidence is in- used as evidence. For example, the inverted stratigraphy is due conclusive and allows for other interpretations. It is interesting to Cenozoic Alpine tectonism, most breccias are due to diage- that other meteorite-impact craters, similar in size and age to netic and/or edaphic processes (evaporite dissolution with col- those proposed for the Azuara structure (Ries in Germany, lapse of host carbonate rocks, karst and caliche development), Haughton in Canada), display numerous impact-related features breccia dikes are also due to karst and paleosol development (impact melts, widespread shock metamorphism) that are cer- on carbonates, and the negative gravity anomaly comes from tainly not observed at Azuara. an incomplete data set restricted to the interior of the Azuara The Azuara structure is located ϳ50 km south of Zaragoza, sedimentary basin, and resulting from its bowl shape. In addi- at the northeastern side of the Iberian Range, close to the Ebro tion, Corte´s and Casas-Sainz (1996) considered that the Azuara basin (northestern Spain; Fig. 2). The present-day structure ob- structure is consistent with a north-south regional shortening served in the Azuara region corresponds to a sedimentary basin during the Tertiary that controlled deformation both in the Var- filled with Tertiary deposits and delimited by folds and thrusts iscan basement and in the Mesozoic-Tertiary sedimentary involving Precambrian-Paleozoic basement and Mesozoic and cover. They interpreted the structure as a synclinal basin located Cenozoic supracrustal rocks. An impact origin for the Azuara over an important depression of the Hercynian basement that structure was interpreted from evidence such as inverted stra- is bounded by a fold and thrust arc in the northern part, and a tigraphy, occurrence of megabreccias and megablocks, breccia poorly defined fold system toward the south. These interpre- dikes, a negative gravity anomaly, and features alleged to be tations refute part of the evidence put forward by Ernstson’s indicative of high-pressure and high-temperature effects (Ernst- group. 556 E. Dıáz-Martıńez, E. Sanz-Rubio, and J. Martıńez-Frıás

Immediately after an impact event, the impact crater is sur- POTENTIAL IMPACT CLASTIC BEDS rounded by a deposit of debris ejected as the result of the col- lision. Most of these ejecta are close to the crater rim, and con- Useful criteria for the recognition of potential impact- tinuous ejecta normally extend about one crater radius from the related units in the sedimentary record are the presence of brec- crater rim, in the case of a nonoblique impact (Melosh, 1989). cia or diamictite beds as probable proximal impact ejecta, and The only unit proposed as probable proximal ejecta related to the presence of spherules as probable distal impact ejecta. This the Azuara structure is the Pelarda Formation (Ernstson and is particularly true when these deposits coincide in time with a ϳ Claudıń, 1990), located 10 km to the south of the supposed well-dated massive extinction event, and/or when they roughly crater rim (Fig. 2), and overlying alluvial fan deposits of the coincide with the age of a known impact event. However, once adjacent Calatayud-Montalbań basin. The origin and age of this identified, the potential of the deposit to be impact related needs unit are debated. Although the Pelarda Formation has been tra- to be proved with unequivocal criteria characteristic of mete- ditionally interpreted as one of the frequent Pliocene-Pleisto- orite impacts: marked geochemical anomalies and shock meta- cene aluvial sedimentary cover units present throughout the morphic features (planar deformation features, diaplectic glass, area (Instituto Tecnolo´gica Geominero de Espanã [ITGE], high-pressure polymorphs). 1989, 1991), Ernstson and Claudıń (1990) and Ernstson and Breccia and diamictite beds are identified in the Spanish Fiebag (1992) interpreted this formation as the remnant of an sedimentary record: our review of the literature revealed that originally extensive ejecta blanket around the Azuara structure. most of them have been interpreted as the result of resedimen- The conglomerates and diamictites of the Pelarda Formation, tation related to slope and/or tectonic instability, and more ϳ 2 which has an outcrop of 30 km , are basically composed of rarely as glacial deposits. Many of these units are clearly related rounded to subrounded quartzite clasts (to 1 m in diameter) to active tectonism or eustacy, although there are some that eroded from the local Paleozoic basement, embedded within a might be impact related. For these, the evidence for a strictly mixed clayey-silty-sandy matrix, and with apparently no inter- terrestrial origin (i.e., unrelated to cosmic impact) is not always nal fabric (Fig. 3B). Carls and Monninger (1974) reported some unequivocal: some features remain to be explained, and alter- Buntsandstein pebbles, but they did not observe limestone com- native hypotheses may relate the deposits to impact events. Fol- ponents. However, Ernstson and Claudıń (1990) added to the lowing is a brief review of the principal characteristics of sev- previous work the identification of Buntsandstein megaclasts, eral units in the sedimentary record of Spain that we have sporadic limestone clasts, and lower Tertiary marls as clasts identified as potential impact clastic beds. Our current research within the conglomerates. Striated and polished boulders and is oriented toward the verification of the terrestrial or impact cobbles of quartzite, schist, and slate were also described by origin of these strata. Ernstson and Claudıń (1990). Plastically deformed and frac- tured clasts, some of them showing rotational deformation, and multiple sets of planar deformation features in quartz were also Vendian-Cambrian boundary described by Ernstson and Claudıń as evidence for shock deformation and metamorphism. However, the deformational fea- Deep-marine breccias and olistostromes known as the tures proposed as evidence for shock metamorphism are unre- Fuentes Bed (Nivel de Fuentes) in the Central Iberian Zone of lated to impact metamorphism (F. Langenhorst, personal the Hercynian Massif broadly coincide with the Vendian- commun., 2000; see also Langenhorst and Deutsch, 1996). Cambrian boundary (location 10 in Fig. 1). They have been On the assumption that the Azuara structure may be an traditionally interpreted as tectonically induced strata, within impact crater, only biostratigraphic and lithostratigraphic meth- the context of the Cadomian or late Pan-African orogeny (San ods help provide an age for the alleged proximal ejecta. This is Jose´, 1984). The unit is present in the Montes de Toledo and because no impact melt sheet or true suevites have ever been Las Hurdes (Alvarez-Nava et al., 1988; Robles and Alvarez- described for the structure, and therefore radiometric methods Nava, 1988) (Fig. 1), whereas it is absent from other areas could not be applied. Cenozoic sediments cover almost the total within the Central Iberian Zone (as in southern Salamanca; surface area of the structure, and there are no deep boreholes. Nozal and Robles, 1988). Along the northeastern flank of the Ernstson and Fiebag (1992) suggested a late Eocene–Oligocene Valdelacasa antiform, near its type locality (town of Fuentes) age for the Pelarda Formation because the Miocene sediments in the central Montes de Toledo, the Fuentes Bed unconform- are not affected by tectonics, and Eocene sediments are incor- ably overlies the deformed deep-marine shales and sandstones porated into some breccia dikes. However, vertebrate paleon- of the Late Proterozoic (Riffean) Domo Extremenõ Group tological data for the units immediately below the Pelarda For- (Alvarez-Nava et al., 1988; Pardo and Robles, 1988; Santa- mation suggest an age younger than early Oligocene (Olalla marıá and Remacha, 1994). To the southeast of the Valdelacasa paleontological site, MP 21 zone; Pelaéz-Campomanes, 1993). antiform, but still within it, the Fuentes Bed overlies both the These data do not exclude a Pliocene-Pleistocene age for the Domo Extremenõ Group and a remnant of the Late Proterozoic Pelarda Formation, which in our opinion can also be interpreted (Vendian) Ibor Group (Santamarıá and Pardo, 1994) (Figs. 3, as local Pliocene-Pleistocene alluvial deposits, which are com- C and D, and 4). Farther to the south of the Valdelacasa anti- mon throughout central Spain along zones of major relief. form, in the Villarta-Navalpino antiform, the Fuentes Bed is Sedimentary record of impact events in Spain 557

Figure 3. A: Microphotograph showing general aspect of Arroyofrı´o Oolite Bed (Dogger-Malm boundary). Cross- polarized light. Scale bar is 1 mm. B: Paleozoic quartzite boulders at southwestern part of Pelarda Formation outcrops. C: General view of Fuentes Bed megabreccia, showing characteristic chaotic aspect. D: Carbonate and quartzite boulders of Fuentes Bed embedded in plastically deformed muddy matrix. Circled hammer for scale. E: Sandstone clast in Orea Formation diamictite in Iberian Range (east of Checa, Guadalajara Province). F: Limestone clast in Orea Formation diamictite in eastern Iberian Range (northwest of Fombuena, Zaragoza Province).

known as the Navalpino Breccia and overlies shallow-marine in situ (autochthonous) sedimentation separating them (Santa- limestones of the Ibor Group (San Jose´, 1984). marıá and Remacha, 1994). The size of the clasts varies from The Fuentes Bed was first described and defined by Mo- millimeter and centimeter size to blocks of several meters, and reno (1974, 1975). In the Valdelacasa antiform it is a rather slabs of 20 to 30 m. The composition is highly polymictic, and continuous bed, between 200 and 300 m thick (Fig. 4). Moreno consists of all the lithologies of the underlying Ibor and Domo (1977) interpreted it as the result of a single event representing Extremenõ Groups, i.e., limestone, dolostone, shale, siltstone, an isochron. However, detailed sedimentologic analysis proved sandstone, conglomerate, and graywacke. Thin sections reveal that it represents a series of multiple resedimentation events the complex character of the matrix, which also includes small (slumps, debris flows, and olistostromes), with no interbeds of lithic igneous clasts of probable volcanic origin, and highly 558 E. Dıáz-Martıńez, E. Sanz-Rubio, and J. Martıńez-Frıás

rare and dubious, whereas no striated pavements or boulder pavements have ever been found. These features remain un- explained, and detailed sedimentological and geochemical studies need to be done.

Late Devonian

Shallow-marine diamictites within the Phyllite-Quartzite Group of the Iberian Pyrite Belt (South Portuguese Zone of the Hercynian Massif) are commonly interpreted as large debris flows related to tectonism (Moreno and Saéz, 1990; Moreno et al., 1995). The age of the Phyllite Quartzite Group in the South Portuguese Zone is not well defined, but it is broadly considered to be of Late Devonian age (Moreno et al., 1995). Sedimentary facies and sequences in the Phyllite Quartzite Group represent storm-dominated shallow clastic shelf deposits, interrupted toward the top by thick (to 60 m), conspicuous beds of massive diamictites. They are common throughout the Iberian Pyrite Belt, but are particularly frequent in the Valverde del Camino antiform, 40 km north of Huelva (Fig. 1). The matrix of the Figure 4. Schematic section of Fuentes Bed at El Membrillar (south- diamictites is abundant (80%–90%) and muddy (shaly). Clasts east Valdelacasa antiform), indicating lithostratigraphic nomenclature within the matrix consist of partially consolidated resedimented and approximate ages. Modified from Alvarez-Nava et al. (1988) and parautochthonous sandstones of variable size, normally reach- Santamarıá and Pardo (1994). Fm.—Formation. ing 20 cm in size. Meter-sized slumps and isolated blocks are also common. deformed quartz clasts, which seem to lack planar deformation Moreno et al. (1995) interpreted these large debris flows features. Many terrigenous clasts display plastic deformation as having been triggered by earthquakes, and related them to and partial disaggregation in the matrix, indicating their incom- tectonic instability, in particular at the beginning of the Her- plete consolidation at the time of resedimentation. Carbonate cynian orogeny in the region. However, mass gravity flows can clasts are thoroughly recrystallized and have been transformed also be triggered by wave loading during strong storms and to dolomite and/or magnesite during diagenesis. tsunami events. Mass-extinction events and meteorite-impact structures are known in the Late Devonian (Sandberg et al., Latest Ordovician 2000), and therefore other possibilities should be considered in the interpretation of the Phyllite Quartzite Group. Shallow-marine, late Ashgill (Hirnantian) diamictites are present in northern Africa and western and southern Europe, Triassic-Jurassic boundary along the former margin of Gondwana, and have been generally interpreted to be coeval with the north African glaciation (For- In most of Spain, the Triassic-Jurassic boundary is marked tuin 1984; Robardet and Dore´, 1988). In Spain, these diamic- by a regional erosional unconformity and/or earliest Jurassic tites are known by different formation names throughout the (Hettangian) breccias (Cortes de Tajunã Formation). In some Central Iberian Zone: pelitas con fragmentos (or fragment- localities, the breccias are interpreted as being related to rifting bearing shales), Gualija Formation, Orea Formation, and and eustacy (Aurell et al., 1992; Gallego et al., 1994; Campos Chavera Formation (Fortuin, 1984; Portero and Dabrio, 1988; et al., 1996), whereas in others they are considered to be the Robardet and Dore´, 1988). Apart from diamictites, these up- result of collapse after evaporite dissolution (Go´mez and Goy, permost Ordovician units also consist of graywacke, shale, 1998). In particular, Campos et al. (1996) described erosion of sandstone, and conglomerate, with a variable total thickness of underlying Triassic units and tectonic collapse of a shallow car- as much as 200 m at some localities. The diamictites include bonate platform of Hettangian age developed during rift exten- reworked clasts and fossils recycled from underlying Ordovi- sion. Go´mez and Goy (1998) identified an evaporite unit (Leć- cian units (e.g., quartz, limestone, shale, sandstone) (Fig. 3, E era Formation) from subsurface data in eastern Spain (Fig. 1), and F), and are overlain by a thin ubiquitous quartzite (Garcıá- coinciding with the Triassic-Jurassic boundary, and consisting Palacios et al., 1996). of 100–200 m of gypsum, anhydrite, and carbonates. This unit The diamictites at most of the sections are resedimented was found to underlie, laterally grade into and replace the Cor- (Portero and Dabrio, 1988). Evidence for a glacial origin is tes de Tajunã Formation in many areas. scarce and inconclusive: glacially striated clasts are extremely One of the most important extinction events of the Phan- Sedimentary record of impact events in Spain 559 erozoic, including 47% extinction of all known marine animal 3. The scarcity of evidence for meteorite-impact events in genera (Sepkoski, 1996), took place near the Triassic-Jurassic the sedimentary record is related to the lack of detailed studies. boundary. At the same time, several meteorite-impact structures We propose several sedimentary units that could potentially be of intermediate and large size are known that have Late related to impact events, and where future research should Triassic-Early Jurassic ages (Rochechouart, Manicouagan, focus. Lake Saint Martin, Red Wing, Obolon), and impact signatures (iridium anomaly, shocked quartz) have been found with them (Montanari and Koeberl, 2000). ACKNOWLEDGMENTS

Cenomanian-Turonian boundary This research is supported by the Spanish Center for Astrobi- ology (Consejo Superior de Investigaciones Cientı´ficas—Insti- Possible impact ejecta have recently been found north of tuto Nacional de Tećnica Aeroespacial). We appreciate helpful Nazare´ (Mesozoic Lusitanian basin of Portugal; Fig. 1), near and constructive reviews of this manuscript by Bruce Simonson the Cenomanian-Turonian boundary, which may be related to and Wolf Uwe Reimold, as well as comments by editor Chris- the Tore Seamount, a possible impact structure located off the tian Koeberl. The IMPACT program of the European Science coast of Portugal (Pena dos Reis et al., 1997; Monteiro et al., Foundation financed the participation of Dıáz-Martıńez and 1997, 1998, 1999). It may be possible to find corresponding Sanz-Rubio in Short Courses on Impact Stratigraphy and Im- distal ejecta in the frequently excellent exposures of shallow- pact Metamorphism (2000), which suggested to us the need for and deep-marine sequences covering this same interval this review, and greatly helped the development of ideas on (Cenomanian-Turonian boundary) in Spain. Some of the Span- prospective units. ish sections are already well dated on the basis of calcareous nannoflora, planktic foraminifera, and ostracod biostratigraphy (Gorostidi and Lamolda, 1991; Gil et al., 1993; Paul et al., REFERENCES CITED 1994; Floquet et al., 1996). 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